This invention relates to the separation and recovery of ethylbenzene from C.sub.8 aromatic isomer feedstreams in a simulated countercurrent flow system wherein a fluid stream containing said feedstream flows through serially and circularly interconnected desorption, rectification and adsorption zones. More particularly, it pertains to the use of certain specific sieves in combination with the dual desorbent composition technique which is carried out by employing two desorbent streams of different strengths to help minimize the total amount of desorbent requirements. Additionally, it concerns the use of said specific sieves and a temperature gradient technique which helps increase the strength of the desorbent employed.
The present process employing the above inventive concepts is an improvement of the simulated countercurrent flow processes described in U.S. Pat. No. 3,761,533 and U.S. Pat. No. 3,201,491. It is known that, in adsorption-separation processes of liquid feed mixtures, the technique of employing a moving-bed type adsorption process, wherein said moving bed comprises adsorbent particles which are counter-currently contacted with streams of liquid feedstock and desorbent, results in a high degree of purity for the adsorbed product. This process and the so-called simulated countercurrent flow system, wherein solid sorbent particles are stationary, have been proposed and disclosed in the above patents.
In the latter known process, an adsorption separation column is divided into three (or four equivalent) zones: a sorption zone, (a primary rectification zone,) a desorption zone and a (secondary) rectification zone. A downstream portion of the sorption zone is also called a primary rectification zone. Each of the three (or four) zones is divided into a plurality of serially interconnected sections, each section being packed with a mass of solid sorbent particles. In such a system, the separation is achieved by: introducing a desorbent stream into the first section of the desorption zone; introducing the liquid feed mixture to the first section of the sorption zone and withdrawing a raffinate effluent comprising a less sorbed component and the desorbent from the sorption zone. In order to maintain the simulated moving-bed operation, all the points of introducing and withdrawing the liquid streams into and from the sections are simultaneously shifted, at stated intervals of time, in a downstream direction, while maintaining the same order of continuity and the same spatial relationship between all the points.
In conducting the above-described process, several attempts were made to reduce the total desorbent requirements and also enhance the purity of the recovered sorbate. Stine et al., for example, disclosed a process in U.S. Pat. No. 3,201,491 (1965) which employs a portion of the desorption effluent withdrawn from the last section of the desorption zone by passing it directly into the (secondary) rectification zone in order to physically wash the raffinate materials remaining in the inactive void interstices between the active sorbent particles. An externally-prepared purging fluid comprising the sorbate and raffinate components of the feedstock was also claimed in their patent. Another improvement described in U.S. Pat. No. 3,455,815 (Fickel; 1967) and U.S. Pat. No. 3,696,107 (Neuzil; 1972) envisions the employment of a stream consisting essentially of an inert material in order to flush non-selectively sorbable components of the feedstock from the interstitial void spaces between the sorbent particles in the rectification zone. A third method of which is described in U.S. Pat. No. 3,761,533 (Otani et al.; 1973) introduces a portion of the desorption effluent which is rich in sorbate content into the rectification zone for the purpose of enhancing the purity of the sorbate component adsorbed within the rectification zone.
The above methods, however, contain certain deficiencies. The use of a portion of the desorption effluent will not only result in an increase in the desorbent consumption but also can desorb certain amount of sorbate adsorbed within the rectification zone and thereby limit the overall efficiency of the system. Further, Fickel's concept of employing a flushing stream consisting essentially of an inert material may not provide an adequate means to desorb chemically-adsorbed raffinate materials; and, consequently, the amount of desorbent requirements may not be lowered. It has now been discovered that the employment of the dual desorbent composition technique and/or the dual temperature technique described herein can substantially eliminate the above deficiencies and markedly improve the preferential separation of the ethylbenzene from C.sub.8 aromatic isomer feedstreams.